Consumers first judge the quality of the food product by its color, at least according to Food Technology, page 49 (July, 1986). The food industry has catered to consumer aesthetics, if not actually fostering such an attitude, by giving careful attention to the color of their products, including conducting ongoing investigations into materials which may be used as suitable food colorants.
Although naturally occurring pigments perforce were the first used food colorants, the development of chemistry as a discipline led to many synthetic dyes, especially anilines, to supplant naturally occurring pigments as food additives. As a class synthetic colorants have many advantages, such as a uniform and reproducible color, color stability, absence of flavor, and an oxidative and/or thermal and/or photostability superior to naturally occurring pigments, broad availability relatively insensitive to changes in crop yields and so forth. The resulting popularity of synthetic colorants at least is understandable.
However, with heightened awareness of a consuming public to food additives and increased testing of some representative examples came a concern about their safety. Recent years have seen some materials formerly used as food colorants run the gamut from being beyond reproach to being suspect and even banned or at least used restrictedly. For example, FD&C Red No. 2 and FD&C Violet No. 1 have been banned in the United States and many other countries. Because of a variety of allergic reactions in sensitive individuals induced by FD&C Yellow No. 5 a recent ruling by the FDA requires food colored with it be declared as such on product labels. As a consequence the pendulum has begun to swing once more toward naturally occurring pigments as food additives.
The major pigments produced by Monascus species traditionally grown on rice in the Orient are orange and relatively insoluble in water, but readily react with compounds containing amino groups to form water-soluble colorants. Monascus pigments have been used in the Orient for hundreds of years as a general food colorant and as a colorant for wine and bean curd. They can be made water soluble or oil soluble and are stable at a pH range 2-10. They are heat stable and can be autoclaved. In oriental countries microorganisms of this type typically are grown on grains of rice and once the grains have been penetrated by the red mycelium the whole mass is finely ground with the resulting powder used as a food colorant.
Monascus species have been reported to elaborate several pigments, but most species seem to produce an orange pigment as the major colorant. This water-insoluble pigment is a mixture of monascorubrin and rubropunctatin, I, whose structures were elucidated by B. C. Fielding et al., Tetrahedron Letters, No. 5, 24-7 (1960) and Kumasaki et al., Tetrahedron, 18, 1171 (1962), and which differ in the former having a 7-carbon chain attached to the ketonic carbonyl group and the latter having a 5-carbon chain. The ratio of monascorubrin and rubropunctatin produced depends on fermentation conditions and the particular species or strain of Monascus used, but generally this ratio is on the order of 3:2. At least some species, notably M. purpureus, produce a yellow pigment, monascoflavin, the reaction product of rubropunctatin with two moles of hydrogen and which arises from reduction of two conjugated olefinic bonds in the chromophore of the parent. Y. Inouye et al., Tetrahedron, 18, 1195 (1962). [Parenthetically, it may be noted that these authors state that monascoflavin is the reduction product of monascorubrin. However, monascorubrin and rubropunctatin are homologs differing in having C.sub.7 H.sub.15 and C.sub.5 H.sub.11 ketonic side chains, respectively, and monascoflavin is specified as having a C.sub.5 H.sub.11 side chain. Therefore its precursor must be rubropunctatin. It must be realized that for many years there was rampant confusion between monascorubrin and rubropunctatin, with a concomitant lack of distinction, whose effects are not yet entirely dispelled.]
Although the monascorubrin-rubropunctatin mixture which constitutes the orange pigment produced as the direct fermentation product of Monascus species is water insoluble and therefore is of limited utility as a food colorant, it has been recognized for some time that these materials react with primary amines to afford red colorants, many of which are water soluble. Yamaguchi, U.S. Pat. No. 3,765,906, reported that the orange insoluble pigment, either in the fermentation medium or as an isolate, reacted with water-soluble proteins, peptides, or amino acids to afford red water-soluble pigment. The reaction of the orange water-insoluble pigment with amino sugars, polymers of amino sugars, polyamino acids, and amino alcohols is reported in U.S. Pat. No. 3,993,789. The production of red water-soluble pigment by reacting the insoluble orange pigment with aminoacetic acid (glycine) and aminobenzoic acid has been reported by Wong and Koehler, J. Food Science, 48, 1200 (1983), who also investigated their color characteristics and stability. All of the aforementioned water-soluble red pigments are believed to have the structure II, ##STR1## where II is a monascorubrin-rubropunctatin mixture and R.sub.1 is C.sub.5 H.sub.11 or C.sub.7 H.sub.15. Despite the interest as manifested by the numerous citations, none of the red water-soluble pigments appear to have gained broad, substantial use as a food colorant.
Mixtures of red, water-soluble pigments resulting from the reaction of protein hydrolysates with I have seen limited use in some countries. However convenient and inexpensive such mixtures may be, their variability presents some problems with color reproduction, and the indefinite nature of their composition complicates the safety, toxicity, and pharmacological/physiological testing necessary for their clearance as food colorants in edible formulations intended for human consumption. The importance of exact knowledge of the structure was recognized by Moll et al., the patentees of U.S. Pat. No. 3,993,789, whose purpose was, in part, to provide red colorants of well-defined structure.
Red colorants of suitable purity for use in foods intended for human consumption, here defined as being at least 95% pure, rarely, if ever, have been produced in the prior art. Thus, Moll et al. use the "pure crystalline form" of what is referred to as monascorubrin to react with various amines to give red, water-soluble pigments whose purity is nondescript. Analogously, Wong and Koehler, op. cit., react purified, recrystallized precursor pigment (whose purity was not determined) with glycine, p-aminobenzoic acid, and L-glutamic acid to give colorants which were not isolated. Kumasaki et al., op. cit., reacted N-methylamine with precursor pigment and purified the product by chromatography to afford a red pigment whose elemental analysis was outside the bounds of the .+-.0.3% variance from theory normally associated with high purity.
In contradistinction to the prior art we have been able to routinely prepare red pigments of appropriate purity. Our success hinges in part on the availability of the water-insoluble orange pigment from Monascus in high purity. The remainder of our success derives from our observation that the monascorubrin-rubropunctatin mixture reacts with approximately stoichiometric quantities of amines having a primary amino group in essentially a quantitative manner and with virtually 100% selectivity. The result is production of red pigments of at least 95 percent purity and with a well-defined structure. In addition, many of these water-soluble red pigments possess no objectionable taste when used in an amount effective to impart a red color in foods designed for human consumption.